Background: Claudins are members of a large family of tight junction proteins, which regulate cellular adhesion, polarity, and glandular differentiation. Dysregulation of claudin protein expression has been described in a number of malignancies, including lung carcinomas; however, expression of claudins in pulmonary neuroendocrine tumors (PNT) has not been addressed. Our goal was to investigate the protein expression patterns of claudins 1,3,4,7and 8 in PNTs.Design: Sixty seven cases of PNTs were retrieved from the archives of Rhode Island Hospital including 38 typical carcinoids (TC), 11 atypical carcinoids (AC), 9 small cell carcinomas (SCC), 4 large cell neuroendocrine carcinomas (LCNEC), and 5 combined small cell/large cell neuroendocrine carcinomas (CSLNEC). Paraffin embedded tissue microarrays were analyzed for IHC expression of claudins 1,3,4,7, and 8. The immunoreactivity was assessed based on a combined score of the extent and intensity on a scale of 0-3+.Results: In the normal lung tissue bronchial epithelial cells exhibited membranous claudin staining for all of the claudins studied. Alveolar cells demonstrated diffuse membranous immunoreactivity for claudins 7 and 8, focal staining for claudins 3 and 4, and were negative for claudin 1. In PNTs all claudins exhibited membranous staining pattern. A relatively low frequency of moderate to strong claudin expression (2-3+) varying from 2.6 to 36.3% was observed for claudins 1,3, and 7 in TC, AC, and SCC. Claudin 8 was weakly expressed in TC and AC, and was negative in all SCC cases. In contrast to other claudins, claudin 4 exhibited significantly higher expression in the more aggressive tumor subtypes as opposed to TC (p<0.001). Moderate to strong (2-3+) claudin 4 expression was detected in 2.6% of TC, 18.2 % of AC, 55.6% of SCC, 75% of LCNEC, and 80% of CSLNEC. Interestingly, while the majority of LCNEC were positive for claudin 4, none were positive for claudins 1,3, and 7 and only one case was positive for claudin 8.

Expression of Claudin Proteins in PNTs

TC (n=38)

AC (n=11)

SCC (n=9)

LCNEC (n=4)

CSLNEC (n=5)

Claudin 1

5.3 %

27.3 %

11.1 %

0.0 %

0.0 %

Claudin 3

10.5 %

18.2 %

22.2 %

0.0 %

20.0 %

Claudin 4 *

2.6 %

18.2 %

55.6 %

75.0 %

80.0 %

Claudin 7

21.6 %

36.3 %

33.3 %

0.0 %

20.0 %

Claudin 8

16.2 %

27.3 %

0.0 %

25.0 %

60.0 %

* P<0.001

Conclusions: This study is the first to comprehensively examine the expression of claudins 1,3,4,7 and 8 in PNTs. Claudins are differentially expressed and overexpression of claudin 4 in SCC, LCNEC, and CSLNEC suggests that this protein may be involved in progression to more aggressive tumor subtypes.Category: Pulmonary